This one is not all that surprising, simply because it is a substance. When you drink a cup of joe in the morning, the caffeine in the coffee travels through your blood and enters your brain. There, it binds to receptors that are designated to deal with adenosine.
Adenosine is what tells your brain to start feeling tired. If this chemical cannot deliver its message, you end up feeling more awake. If we deliver regular doses of any chemical to our brains for an extended period, the brain becomes dependent upon this chemical.
It stops being able to function properly without it. This is what we refer to as an addiction. In the context of caffeine, it does not necessarily mean that you can’t stay awake without it. But, when you try to cut out coffee, you start noticing the addiction.
You may find your ability to concentrate compromised. It’s also possible to become irritable and unnecessarily tired. The good news is that if you can cut coffee out for a full two weeks, your brain will reset to what it was like before caffeine entered the picture.